1. Field of the Invention
The invention relates to lithium-drifted silicon Si(Li) x-ray detectors, and in particular to the shape of such detectors.
2. Related Art
X-ray detectors of this type traditionally have two shapes.
The first prior art shape is depicted in FIG. 1. FIG. 1 is a cross-section. The detector is rotationally symmetric about an axis A-B shown in the figure and is made of the well-known material, lithium-drifted silicon. X-rays incident on front surface 103 are converted to an electric current by the detector held under voltage bias. Current is drawn off the detector from back surface 104 into a detector assembly for amplification and further processing.
The shape of this traditional detector is known as a "top hat structure" because of its resemblance to a historical nineteenth century hat. This structure has the advantage of being relatively cheap to manufacture.
A second prior art shape is depicted in FIG. 2. This shape is also rotationally symmetric about the axis A-B. The shape is a cylinder characterized by a groove 201 running concentrically about the axis A-B. This structure is referred to herein as the "deep-grooved structure".
The deep-grooved structure has several advantages over the top hat structure. The high outer rim 203 protects the inner side surface 202 from mechanical damage due to handling. The groove 201 reduces the mass of the detector, improving energy resolution.
The center portion of both structures is the lithium compensated active area of the detector. Diffusion of lithium from this area occurs readily at room temperature. The groove 201 inhibits diffusion of lithium from the central portion 210 of the detector to the outer portion, increasing service life.
In addition, conductive contaminants are less likely to be deposited in the groove 201 than on other surfaces of the detector. The resultant gap in conductive contaminants reduces current leakage between the front 103 and back 104 surfaces of the detector during operation, when a voltage difference is maintained between these surfaces.
The deep-grooved structure has several disadvantages.
It has significantly higher manufacturing costs than the top hat structure. Handling is difficult when the inner side surface 202 is the same height as the outer rim 203. Moreover, chemical etching and polishing of all surfaces except the back surface 104 is often necessary to correct mechanical damage due to handling. The narrowness of the groove 201, normally between 0.5 and 0.7 mm, makes the etch rate at the bottom of the groove 201 difficult to control. This results in low production yield.
Furthermore, in normal operation, surfaces 100, 101, 102, 202 and 203 and the groove 201 are protected with a passive layer. Since the detector is costly, when a detector fails to meet requirements during production or in normal use, the failed detector may be reprocessed chemically to put on a new passive layer. All surface must be cleaned to remove the passive layer before reprocessing to prevent cross-contamination which would make production yield even lower. Extensive labor is required to clean groove 201, no matter what cleaning process is used.
It has also been known to make a detector which is not rotationally symmetric, where necessary to fit into available space. Such detectors traditionally have the appearance of the top-hat or deep-grooved structure with a piece cut off, typically beyond some plane parallel to the axis A-B.
Prior art detectors are known particularly from F. S. Goulding et al., "Detector Background and Sensitivity of Semiconductor X-ray Fluorescence Spectrometers", Advances in
X-ray Analysis, Vol. 15, 1972, pp. 470-482. General background information about x-ray detectors can be found in publications such as F. S. Goulding and J. M. Jaklevic, "Photon-excited Energy-dispersive X-ray Fluorescence Analysis for Trace Elements", Annual Review of Nuclear Science, Vol. 23, 1973, pp. 45-74; N. W. Madden, et al., "An Improved Operating Mode for a Si(Li) X-ray Spectrometer", IEEE Transactions on Nuclear Science, Vol. 37, 1990, pp.171-176; and U.S. Pat. No. 4.931,650.